This application claims priority to Japanese patent application Nos. JPAP11-260935 filed on Sep. 14, 1999, JPAP11-187748 filed on Jul. 1, 1999, and JPAP2000-163315 filed on May 31, 2000, the entire contents of which are hereby incorporated by reference.
1. Field
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of changing a fixing temperature in accordance with variations of a speed at which a recording sheet is transferred.
2. Description of the Related Arts
A high image resolution has recently been an important feature for image forming apparatuses such as printers, copying machines, facsimile machines. Various kinds of improvements have accordingly been made on an optical writing system of such an image forming apparatus. An image forming apparatus capable of forming an image with different image resolutions at a sheet transfer speed exists but has problems of a complex controlling feature and, accordingly, a relatively high manufacturing cost. In order to avoid these problems, some image forming apparatuses change a sheet transfer speed in accordance with the variations of the image resolution. For example, the sheet transfer speed is reduced in half when the image resolution is doubled.
In addition, many of the above-mentioned image forming apparatuses use a heat roller and a pressure roller for the image fixing operation in which a toner image is heated and fixed with a pressure on a recording sheet such as a paper sheet, an OHP (overhead projector) sheet, and the like. During the image fixing operation, a heating value of the heat roller per unit time is controlled so that the heat roller maintains a predetermined fixing temperature. Thereby, the heat roller can give an appropriate amount of heat to a recording sheet at a nip position formed between the heat pressure rollers. When an excess amount of the fixing heat is given to the toner image, toner particles may be melt which may cause a spot-off phenomenon in which a weak black spot appears in a black area and which may also generate wrinkles. When a less amount of the fixing heat is given to the toner image, the fixing operation may be defective.
As described above, in an image forming apparatus in which a change of the image resolution is achieved by changing the sheet transfer speed, an amount of the fixing heat is varied in accordance with the changed sheet transfer speed and, therefore, the spot-off and wrinkles may be generated. To avoid these problems, it may be effective to set a higher goal value to which the fixing temperature is increased when a faster sheet transfer speed is applied (i.e., a lower image resolution) and to set a lower goal value when a slower sheet transfer speed is applied (i.e., a higher image resolution) so as to continuously provide an appropriate heat to the fixing operation.
However, by the above-mentioned way of avoiding the problems, the variations of the sheet transfer speed simply causes a problem of a bad production or a problem in which predefined controls of the fixing operations, particularly for a multi-resolution type image forming apparatus, is damaged. In addition, if the goal value of the fixing temperature is changed in an image forming apparatus, the fixing operation may be completed when the heat roller is heated up to the temperature of the goal value. In this case, the fixing operation is performed with an improper fixing temperature and causes a production in a bad print quality.
The present invention provides a novel image forming apparatus. In one example, a novel image forming apparatus includes a fixing mechanism, a power source, a sheet transfer mechanism, and a controller. The fixing mechanism includes a heater, and fixes a toner image on a recording sheet at a fixing position. The power source drives the heater. The sheet transfer mechanism transfers the recording sheet from a sheet cassette to an eject tray via the fixing position. The controller changes a sheet transfer speed upon varying an image resolution, instructs the power source to drive the heater to increase a temperature of the heater up to a goal temperature. Further, the controller sets the goal temperature to a predetermined degree when the sheet transfer speed is greater than a predetermined speed, and controls the temperature of the heater via the power source by varying the goal temperature in accordance with values of the sheet transfer speed when the sheet transfer speed is smaller than the predetermined speed.
The controller may change the goal temperature to one of at least three different values.
The controller may set the goal temperature to a first temperature when the sheet transfer mechanism transfers the recording sheet at a first speed and to a second temperature when the sheet transfer mechanism transfers the recording sheet at a second speed. In this case, the first temperature is higher than the second temperature and the first speed is faster than the second speed. Further, the controller instructs the sheet transfer mechanism to start a sheet transfer operation when a temperature of the heater reaches the goal temperature changed in accordance with the sheet transfer speed when the controller changes the sheet transfer speed.
The sheet transfer mechanism may include a registration member for holding and registering the recording sheet before entering into the fixing position.
The controller may instruct the sheet transfer mechanism to start a sheet transfer operation at a time so that a recording sheet arrives at the fixing position when a temperature of the heater reaches the goal temperature which is changed in accordance with the sheet transfer speed when the controller changes the sheet transfer speed.
The image forming apparatus may further include a mode selection mechanism for selecting one of first and second modes. In the first mode, the controller instructs the sheet transfer mechanism to start a sheet transfer operation after a temperature of the heater reaches the goal temperature which is changed in accordance with the sheet transfer speed when the controller changes the sheet transfer speed. In the second mode, the controller instructs the sheet transfer mechanism to start a sheet transfer operation before a temperature of the heater reaches the goal temperature which is changed in accordance with the sheet transfer speed when the controller changes the sheet transfer speed.
In the first mode, the controller may instruct the sheet transfer mechanism to start the sheet transfer operation at a time so that the recording sheet arrives at the fixing position when the temperature of the heater reaches the goal temperature. Further, in the second mode, the controller may instruct the sheet transfer mechanism to start the sheet transfer operation at a time so that the recording sheet arrives at the fixing position before the temperature of the heater reaches the goal temperature.
The mode selection mechanism may select one of the first and second modes when the sheet transfer speed is changed from the first speed to the second speed.
The mode selection mechanism may independently select one of the first and second modes when the sheet transfer speed is changed from the first speed to the second speed and when the sheet transfer speed is changed from the second speed to the first speed.
The mode selection mechanism may select one of the first and second modes based on a command sent from an external host system connected to the apparatus.
The apparatus may handle thick and thin recording sheets and the mode selection mechanism may select the first mode for the thin recording sheet.
Further, the present invention provides a method for image forming which includes the steps of changing, setting, controlling, increasing, and fixing. The changing step changes a sheet transfer speed for transferring a recording sheet upon varying an image resolution. The setting step sets a goal temperature to a predetermined degree in accordance with values of the sheet transfer speed when the sheet transfer speed is greater than a predetermined value. The controlling steps controls a temperature of a heater when the sheet transfer speed is smaller than the predetermined speed. The increasing step increases the temperature of the heater up to the goal temperature. The fixing step fixes a toner image deposited on the recording sheet at the goal temperature.